1. Field of the Invention
The invention relates to a method for adjusting a projection objective of a projection exposure machine for microlithography for the purpose of fabricating semiconductor components having a number of optical elements, which can be set via manipulators, for simultaneously minimizing a number of aberrations of the projection objective, the minimization of the aberrations being carried out by manipulating at least one portion of the optical elements with the aid of their respective manipulators.
2. Description of the Prior Art
EP 1 231 516 A2 discloses a method for specifying, fabricating and adjusting a projection objective. For the specification, that is to say the description of the optical properties of the projection objective, use is made in this case of a description of the transmission function of the objective pupil for a number of field points. Field points represent a specific position in the object or image plane of the projection objective. The scalar transmission function of the objective pupil can be specified for each field point in the form of a two-dimensional complex variable. The phase of this complex variable is also denoted wave aberration. EP 1 231 516 A2 describes these wave aberrations for each field point by means of so-called Zernike coefficients. Consequently, the image-forming properties of the projection objective are likewise specified by the specification of these Zernike coefficients.
In order to ensure optimum use of the projection objective in a projection exposure machine for microlithography, for example for the purpose of fabricating semiconductor components, the above-described specification of the image-forming properties of the projection objective is very important—although, of course, in addition to accurate knowledge relating to the lithographic process to be carried out with the aid of the projection exposure machine (illumination, precision of the structures to be exposed, photo-resist process, etc.). In general, it is not only a single lithographic process which is relevant, but rather it must even be possible to carry out a multiplicity of various lithographic processes with the aid of the projection objective. In order for it to be possible to find a relationship between this multiplicity of lithographic processes and the properties of the projection objective, their most general description of the image-forming properties of the projection objective is sensible.
The relationship between the objective properties and the lithographic process is established in EP 1 231 516 A2 with the aid of these Zernike coefficients. This can be accomplished in many cases with the aid of a linear model, given the assumption of sufficiently small aberrations.
Following on from the fabrication of a projection objective, a concluding optimization by means of the manipulators (xy-manipulation, tilt manipulation, z-manipulation, wavelength, gas pressure or reticle tilt and reticle height) of the optical elements located in the projection objective is important in deciding the final image forming quality or the image-forming properties of the projection objective.
It is known to introduce slight changes in the optical properties by measuring parameters for which the effects of the manipulation for the optical elements on these parameters are known, whereupon optimization of the parameters is carried out. As described at the beginning, Zernike coefficients which describe the image-forming properties of the projection objective are determined as a rule for this purpose from measured values. This is achieved, for example, by means of measurements at a number of field points in the field, relevant for lithographic imaging, in the image plane of the projection objective. Zernike coefficients with designations Z2 to Z37 are determined in this way (compare EP 1 231 516 A2), after which the optimization is performed. The average root-mean-square deviation of all the measured field points from 0 is minimized for this purpose in the case of each Zernike coefficient (so-called least square optimization).